The short answer is no. Replacing the volume pot with a lower value pot will only worsen the situation.

Basically you haven't got enough gain in the system (insufficient voltage) or you have insufficient power driving the speakers. This is odd. Most modern sources (CD players, MP3 players) have ample output voltages, and you have a preamp anyway, which is intended to provide some signal amplification from e.g. a magnetic cartridge to drive a power amplifier.